1. Field of the Invention
The present invention relates to an exhaust gas purifying catalyst for purifying exhaust gases emitted from an internal combustion engine by removing NOx therein.
2. Description of the Related Art
As internal combustion engines which are advantageous in reducing fuel consumption, there are known lean burning internal combustion engines on which an air-fuel ratio is controlled to be on the lean side of the stoichiometric air-fuel ratio and direct injection internal combustion engines in which fuel is injected directly into combustion chambers for lean burning (hereinafter, referred to generally as lean-burn engines). In the lean-burn engine which runs lean on stoichiometric (lean operation) to improve fuel economy, an exhaust gas purifying catalyst (a NOx trapping catalyst) is provided for purifying exhaust gases through reduction of NOx (oxides of nitrogen) therein.
According to related-art NOx trapping catalysts, there have been proposed various NOx trapping catalysts (for example, refer to JP-A-2006-26635). These NOx trapping catalysts are made as a catalyst having characteristics in which NOx in exhaust gases is adsorbed as nitrate X—NO3 in an oxidation atmosphere (a lean air-fuel ratio) in which the concentration of reductants is low and the adsorbed NOx is reduced to N2 in a reduction atmosphere (a stoichiometric air-fuel ratio or a rich air-fuel ratio) in which a large amount of reductants such as CO (carbon monoxide) and HC (hydrocarbons) exists.
In the related-art NOx trapping catalyst of this type, NOx in exhaust gases is adsorbed to be prevented from being emitted into the atmosphere by the engine running with the lean air-fuel ratio, and the adsorbed NOx is then desorbed for reduction by controlling the air-fuel ratio to be shifted to the rich side periodically. In order to obtain such a function, in the NOx trapping catalyst, catalyst layers are supported in a honeycomb substrate formed from a ceramic material, and the catalyst layers each contain, for example, noble metals such as platinum (hereinafter, abbreviated to Pt), palladium (hereinafter, abbreviated to Pd), and rhodium (hereinafter, abbreviated to Rh), a heat-resistant inorganic oxide and an alkaline metal or alkaline earth metal as a NOx trapping material.
In recent years, in the lean-burn engines, in order to exhibit the merit thereof in terms of fuel economy, efforts have been made to expand the lean operation range. Because of this, there have been developed various NOx trapping catalysts which can obtain a high NOx removing or reducing performance even in the event the lean operation range is expanded. On the other hand, since expensive noble metals are used in the NOx trapping catalysts for adsorbing NOx as nitrate, the NOx trapping catalysts are regarded as one of expensive automotive parts.
Currently, due to the aforesaid situations, there exists a demand for NOx trapping catalysts which can maintain their maximum NOx reducing performance while concurrently suppressing the production costs by improving the current utilization state of the expensive noble metals. Although such a utilization state of noble metals is disclosed in the related technology described in JP-2006-26635, the technology discloses nothing about a way of using noble metals which can make production costs compatible with performance.